hwmon: qpnp-adc-tm : Use gain parameters for reverse scaling

ADC_TM uses different reverse scaling functions for different
prescaling values. Use gain parameters such that we can have a
generic reverse scaling function for different prescaling values.

Change-Id: I1254580ed985bb825c3371b804d7d4a7e032241e
Signed-off-by: Rama Krishna Phani A <rphani@codeaurora.org>